(A) Field of the Invention
This invention relates to the preparation of purified cresidine sulfonic acid suitable for use as an intermediate in food grade materials. More particularly, this invention relates to the removal of residual cresidine from impure cresidine sulfonic acid.
(B) History of the Prior Art
Cresidine sulfonic acid is usually prepared by sulfonation of p-cresidine to form the p-cresidine sulfate salt which rearranges to form p-cresidine sulfonic acid. p-Cresidine, also known as 2-methoxy-5-methylaniline or 5-methyl o-anisidine, is a compound known in the art which is manufactured by known procedures. One such procedure, for example, is by the reduction of 4-methyl-2-nitroanisole.
Cresidine sulfonic acid prepared by sulfonation of p-cresidine usually contains some residual cresidine. Such cresidine can be removed by established procedures to as low as a few parts per million or lower. Cresidine, however, is completely undesirable in p-cresidine sulfonic acid when the p-cresidine sulfonic acid is to be used in the manufacture of ingestible substances such as food dyes. p-Cresidine sulfonic acid is, for example, used as an intermediate in the manufacture of FDC Red No. 40.
While it has not been established that very low levels of cresidine such as one part per million cause any substantial health hazard, it is desirable to remove as much cresidine as possible from p-cresidine sulfonic acid which is used as an intermediate in the manufacture of ingestible substances since cresidine might be a potential carcinogen at higher concentrations.
In the prior art, it was not possible to remove cresidine to below detectable limits, i.e., about two parts per billion or lower.